Recently, digital terrestrial broadcasting service using the orthogonal frequency division multiple (hereinafter abbreviated as “OFDM”) method has been started. Further planned is digital terrestrial broadcasting service for mobile devices that can be viewed and listened to even by a portable receiving terminal.
In the portable receiving terminal, a small antenna having a reception position lower than that of a large antenna generally for domestic use receives a broadcast. In addition, because the user of the portable receiving terminal views or listens to a broadcast while moving, its reception environment is more considerably deteriorated than that of a domestic television receiver. For this reason, in the portable receiving terminal, it is an important problem to restore the waveforms of transmission signals from the waveforms of received signals and reproduce broadcast programs, in poor reception environments having frequency-selective strains of transmission paths caused by multipath or phasing caused by reception during moving.
It has conventionally been known that diversity reception technology is effective in such poor reception environments. For example, in a space-diversity reception system, two antennas are installed to be separated spatially enough. Power of the signals received by each antenna is compared, and the signals are selected or synthesized. In this manner, the waveforms of the signals at transmission can be restored, even when the power received by one of the antennas is low, but the power received by the other antenna is high.
The OFDM method is a method of transferring signals using a plurality of carrier waves (hereinafter abbreviated as “subcarrier”) orthogonal to each other. For this purpose, in the OFDM method, the signals received by the respective antennas undergo the Fourier transform to be decomposed into subcarriers on a frequency axis, and selected or synthesized for every subcarrier. This can significantly reduce deterioration caused by frequency-selective strains of transmission paths.
Japanese Patent Unexamined Publication No. 2001-156738 discloses an OFDM diversity receiver capable of performing effective diversity so that error correction characteristics are improved even when specific subcarriers are interfered.
However, in the diversity reception system of selecting and synthesizing signals for each subcarrier, because the signals received by antennas are transformed into signals for each subcarrier, tuners and digital demodulators in the same number as antennas are necessary. This structure more considerably increases power consumption than a single reception method using one antenna. Because most portable terminals for receiving digital broadcasting service are powered by batteries, minimizing the power consumption is a large problem of the diversity reception.